This invention relates to photoflash lamps and, more particularly, to flashlamps containing a combustible material which is ignited to produce actinic light.
A typical photoflash lamp comprises an hermetically sealed glass envelope containing a quantity of combustible metal, such as shredded zirconium or hafnium foil, and a combustion supporting gas, such as oxygen, at a pressure well above one atmosphere. In lamps intended for battery operated flash systems, the envelope also includes an electrical ignition system comprising a tungsten filament supported on a pair of lead-in wires having a quantity of ignition paste on the inner ends thereof adjacent to the filament. This type of lamp is operated by the passage of an electrical current through the lead-in wires which incandesces the filament to ignite the ignition paste which in turn ignites the combustible metal in the envelope. In the case of percussive-type photoflash lamps, such as described in U.S. Pat. No. 3,535,063, a mechanical primer is sealed in one end of the lamp envelope. The primer may comprise a metal tube extending from the lamp envelope and a charge of fulminating material on an anvil wire supported in the tube. Operation of the percussive photoflash lamp is initiated by an impact onto the tube to cause deflagration of the fulminating material up through the tube to ignite the combustible metal disposed in the lamp envelopes.
During lamp flashing, the rapid combustion process causes molten droplets of metal and metal oxide to be ejected from the burning strands of combustible metal and to impinge upon the inner walls of the glass envelope. The radiative energy of these molten droplets is directly related to the light output characteristics of the flashlamp, and it has long been recognized that each droplet-wall collision results in a substantial loss of radiative energy from the molten droplet with an attendant diminishing of light output. For example, U.S. Pat. No. 3,377,126 of Nijland et al views this problem from the standpoint of the light absorbing deposits which collect on the inner wall of the envelope during flashing. After the molten droplets collide with the glass wall, there often remains gray or blackish deposits which appear as wall encrustations of approximately 0.2 mm to 1.0 mm in diameter, apparently the result of non-quantitative combustion products. Such deposits can clearly reduce light output by masking or optically attenuating radiation within the lamp vessel. Further, the deposits contain incompletely reacted zirconium or hafnium which is lost for the generation of useful light. The problem is compounded by the current trend to smaller lamp envelope sizes, as the resulting shorter paths of travel for ejected molten droplets mean the resulting droplets will be even less completely burned out prior to colliding with the envelope wall. The use of excess oxygen to alleviate these problems has generally proved ineffective as a remedy.
The Nijland et al. U.S. Pat. No. 3,377,126 attacks the problem by proposing the use of colorless inner wall coatings which release gaseous dissociation products which react with the combustible material "so that the bulb wall cannot be affected or darkened by incompletely burned reducing reaction products". Gains in brightness of over 20% are claimed for applications of multiple coatings of inorganic substances and organic polymers. Suitable inorganic substances proposed are colorless oxygen-releasing compounds such as nitrates, chlorates and perchlorates; whereas the proposed organic substances are colorless, polymeric, fluorated hydrocarbon compounds which evaporate or dissociate at a relatively low temperature. Hence, it appears that the Nijland et al patent provides a vapor or oxygen "cushion" about the inner wall which apparently delays droplet-wall collisions and the condensation or combustion products on the envelope wall.
The most obvious disadvantages of the application of organic coatings, as proposed by the Nijland et at patent, is the possible danger of increasing internal lamp pressures during flashing, expecially in miniaturized lamps with volumes less than 1 cc. These small vessels typically contain fill pressures in excess of five atmospheres and any additions of hydrocarbons would certainly increase the probability of containment failure. Additions of inorganic substances, such as the evaporable solutions of perchlorates suggested by the Nijland patent, would add to lamp manufacturing difficulties and pose problems with retainment of water during lamp processing which would seriously affect light output characteristics.
U.S. Pat. No. 3,630,650 of Kaufmann et al., on the other hand, views the problem of radiative energy losses as due to the heat sink effect of the glass with respect to the burning metal strands and controls shred configuration to minimize these undesired energy losses. More specifically, the Kaufmann et al patent proposes the use of crumpled shreds to promote "optimum combustion without energy losses due to extensive heat transmission to the glass". The process of crumpling provides sharp bends in the shreds which are intended to reduce the bearing area of the combustible material on the inner wall of the lamp envelope. Light output gains of up to 40% are claimed. Nevertheless, the Kaufmann approach still prevents only a fraction of the incompletely reacted droplets from striking the inner lamp walls. Our measurements have shown that losses in brightness of approximately 40-50% occur via wall collision losses, even in the case of flashlamps employing the teachings of Kaufmann.